The present invention relates to a discharge exciting pulse laser device such as an eximer laser device. More particularly the invention relates to a pulse generator for use in a discharge exciting pulse laser device.
FIG. 1 is a circuit diagram showing a conventional eximer laser device described in "OPTICS COMMUNICATIONS", Vol. 56, No. 1, Nov. 1, 1985, p.51. In the figure, reference numeral 1 designates a laser chamber filled with a laser gas, such as XeCl, and containing electrodes and the like as described below. Reference numeral 2 designates a first main electrode with a convexly curved discharge surface; 3, a second main electrode made of a curved mesh metal, which is disposed facing the first main electrode 2; 4, an auxiliary electrode disposed within an concave portion of the second main electrode 3; 5, an insulating member disposed so as to cover the surface of the auxiliary electrode 4; 6, a charge terminal; 7, a charge resistor; 8, a switch including a spark gap whose one pole is connected to the second main electrode and is grounded; 9, a first charge capacitor coupled in series with a reactor 10 between the other pole of the switch 8 and the first main electrode 2; 11, a charge capacitor connected between the second main electrode 3 and a node of the first charge capacitor 9 and the reactor 10; 12 a second charge capacitor connected to the other pole of the switch 8 and the auxiliary electrode 4; 13, a peaking capacitor connected between the main electrodes 2 and 3; 14, a resistor as a first charge circuit element connected between the main electrodes 2 and 3; 15, a resistor as a second charge circuit element connected between the second main electrode 3 and the auxiliary electrode 4.
The operating of the pulse generator thus arranged will be described. A power source (not shown) is turned on to supply a DC voltage to the charge terminal 6. The voltage charges the capacitors 9, 11, and 12, through the charge resistor 7. Since the resistors 14 and 15 are connected between the electrodes, a satisfactory voltage is applied to the capacitors 9, 11, and 12.
The switch 8, the reactor 10, and the capacitors 9 and 11 constitute a conventional LC inverter. With the LC inverter, if the switch 8 at the spark gap after the charge is completed, the voltages appearing across the capacitors 9 and 11 are superposed one upon the other, as indicated by a waveform 1 in FIG. 2, and a high pulse voltage appears between the main electrodes 2 and 3. Simultaneously with the close of the switch 8, the second charge capacitor 12 also is discharged, and a pulse voltage as indicated by waveform 2 in FIG. 2 appears across between the second main electrode 3 and the auxiliary electrode 4.
The generation of the pulse voltage causes a corona discharge 16 to occur between the main electrode 3 and the auxiliary electrode 4. As a result, ultraviolet light due to the corona discharge pass through the second electrode 3 of the mesh structure so as to irradiate the gas in the space between the main electrodes 2 and 3. A preliminary ionization is caused in the space by the irradiation. The insulating member 5 is provided to for prevent the corona discharge from transforming 16 to an arc discharge.
As the crest or peak value of the pulse voltage between the main electrodes 2 and 3 increases, electrons generated through the preliminary ionization serve as seeds to cause impact ionization. Then, a main discharge 17 occurs between the main electrodes 2 and 3 resulting in the occurrence of a laser oscillation. The peaking capacitor 13 is provided to increase the peak value of the voltage between the main electrodes 2 and 3 due to its capacitor nature, or to increase the peak power to the main discharge 17.
If a rise of the voltage between the second main electrode 3 and the auxiliary electrode 4 is made quick, the preliminary ionization due to the corona discharge 16 is facilitated, as a result of which the main discharge 17 is more uniform and the laser output power is thus increased. The above fact has been disclosed in "J. Appl. Phys." 54(10), Oct. 1983, pp 5672 to 5675. The rising speed of the pulse voltage in the circuit of the auxiliary electrode 4 depends greatly on stray inductance components of the circuit. Particularly as for the stray inductance and resistance components at the terminal of the switch 8, the current of the circuit of the main discharge 17 may be allowed to flow thereinto. This results in a voltage drop, which in turn delays the rising of the voltage in the circuit of the corona discharge 16.
Accordingly, if the current flowing in the circuit of the main discharge 17 is restricted by increasing the reactance of the reactor 10, the rise of the voltage in the circuit of the corona discharge 16 is made proportionally rapid (see waveform 2 of FIG. 3).
With such a discharge exciting pulse laser device thus assembled, if the reactance of the reactor 10 is increased to facilitate the preliminary ionization by the corona discharge, the rise of the voltage pulse in the main discharge circuit becomes slow in speed as indicated by waveform 1 of FIG. 3. Accordingly, the V-t characteristic of the main discharge circuit causes the discharge start voltage V.sub.B to drop, and hence the injection energy is decreased. Consequently, the laser output cannot be increased.